PEORIA — The firework mishap that injured about a dozen people at Glen Oak Park on Thursday was a delayed explosion, not a projectile fired directly into the crowd, according to authorities.

A total of eight to 12 people reported minor burns or other injuries after the firework exploded at the finale of the annual July 3 display, though they required no treatment or received treatment by paramedics at the scene.

Two people were transported by ambulance to an area hospital for treatment of their injuries, said Peoria police Special Operations Lt. Steve Roegge. Roegge said the injuries were minor, and there were no trauma patients with fireworks injuries at OSF Saint Francis Medical Center on Friday afternoon.

Initial emergency calls from the scene of the explosion indicated it was a “mass-casualty” event, but that was quickly determined not to be true when the first Peoria Park District officers arrived at the location, Roegge said.

The timing of the mishap only added to the confusion.

“It was right at the end of the event,” Roegge said.

A Fire Department investigator determined a five-inch mortar launched into the air as designed as part of the official fireworks display, but the mortar experienced a delayed ignition of the main charge. It exploded after descending over the lagoon closer to the crowd of spectators on the other side of the water.

Police from multiple departments also responded to several altercations on and off the park grounds throughout the night, according to police and dispatch records.

Some groups willingly dispersed after police arrived, but pepperballs were used later in the night to break up a fight in the area of Knoxville and Arcadia avenues.

A strong-arm robbery in which the suspect was injured and later taken into custody also was reported about 1:10 a.m. Friday in the 800 block of East Arcadia Avenue.

Matt Buedel can be reached at 686-3154 or mbuedel@pjstar.com. Follow him on Twitter @JournoBuedel.